Quorum-based power-down of unresponsive servers in a computer cluster

ABSTRACT

A quorum-based server power-down mechanism allows a manager in a computer cluster to power-down unresponsive servers in a manner that assures that an unresponsive server does not become responsive again. In order for a manager in a cluster to power down servers in the cluster, the cluster must have quorum, meaning that a majority of the computers in the cluster must be responsive. If the cluster has quorum, and if the manager server did not fail, the manager causes the failed server(s) to be powered down. If the manager server did fail, the new manager causes all unresponsive servers in the cluster to be powered down. If the power-down is successful, the resources on the failed server(s) may be failed over to other servers in the cluster that were not powered down. If the power-down is not successful, the cluster is disabled.

CROSS-REFERENCE TO PARENT APPLICATION

This patent application is a continuation of U.S. Ser. No. 10/981,020filed on Nov. 4, 2004, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention generally relates to data processing, and morespecifically relates to networked computer systems.

2. Background Art

Since the dawn of the computer age, computer systems have becomeindispensable in many fields of human endeavor including engineeringdesign, machine and process control, and information storage and access.In the early days of computers, companies such as banks, industry, andthe government would purchase a single computer which satisfied theirneeds, but by the early 1950's many companies had multiple computers andthe need to move data from one computer to another became apparent. Atthis time computer networks began being developed to allow computers towork together.

Networked computers are capable of performing tasks that no singlecomputer could perform. In addition, networks allow low cost personalcomputer systems to connect to larger systems to perform tasks that suchlow cost systems could not perform alone. Most companies in the UnitedStates today have one or more computer networks. The topology and sizeof the networks may vary according to the computer systems beingnetworked and the design of the system administrator. It is very common,in fact, for companies to have multiple computer networks. Many largecompanies have a sophisticated blend of local area networks (LANs) andwide area networks (WANs) that effectively connect most computers in thecompany to each other.

With multiple computers hooked together on a network, it soon becameapparent that networked computers could be used to complete tasks bydelegating different portions of the task to different computers on thenetwork, which can then process their respective portions in parallel.In one specific configuration for shared computing on a network, theconcept of a computer “cluster” has been used to define groups ofcomputer systems on the network that can work in parallel on differentportions of a task.

Clusters of computer systems have also been used to providehigh-reliability services. The high reliability is provided by allowingservices on a server that fails to be moved to a server that is stillalive. This type of fault-tolerance is very desirable for manycompanies, such as those that do a significant amount of e-commerce. Inorder to provide high-reliability services, there must be some mechanismin place to detect when one of the servers in the cluster becomesinoperative. One known way to determine whether all the servers in acluster are operative is to have each server periodically issue amessage to the other servers indicating that the server that sent themessage is still alive and well. These types of messages are commonlyreferred to in the art as “heartbeats” because as long as the messagescontinue (i.e., as long as the heart is still beating), we know theserver is still alive.

In the prior art, when a server becomes invisible due to lack of aheartbeat, a server in the cluster that is designated as a managerassumes the server that no longer has a heartbeat has failed. As aresult, the manager must provide the resources that were on the failedserver on another server in the cluster. Note, however, that the absenceof a heartbeat does not always mean a server is dead. For example, aserver may not provide a heartbeat because it may be temporarilyunresponsive due to trashing, swapping, network floods, etc. If theserver is not giving heartbeats but is still alive, there exists thepossibility that the server may once again become responsive and startproviding heartbeats. If the manager has already assumed the server hasfailed, and has provided the server's services on another server, we nowhave two servers that try to provide the same services. This creates aproblem in administrating the cluster. One way to deal with this problemis to monitor data for a service to make sure that two servers don't tryto access the same data for the same service. However, this is complexand inefficient. Without a mechanism for assuring that services in acomputer cluster are not duplicated when a server failure is detected,the computer industry will continue to suffer from inadequate andinefficient ways of handling a failed server in a computer cluster.

DISCLOSURE OF INVENTION

An apparatus and method provide a quorum-based server power-downmechanism that allows a manager in a computer cluster to power-downunresponsive servers in a manner that assures that an unresponsiveserver does not become responsive again. In order for a manager in acluster to power down servers in the cluster, the cluster must havequorum, meaning that a majority of the computers in the cluster must beresponsive. If the cluster has quorum, and if the manager server did notfail, the manager causes the failed server(s) to be powered down. If themanager server did fail, the new manager causes all unresponsive serversin the cluster to be powered down. If the power-down is successful, theresources on the failed server(s) may be failed over to other servers inthe cluster that were not powered down. If the power-down is notsuccessful, the cluster is disabled.

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

The preferred embodiments of the present invention will hereinafter bedescribed in conjunction with the appended drawings, where likedesignations denote like elements, and:

FIG. 1 is a block diagram of a computer apparatus in accordance with thepreferred embodiments;

FIG. 2 is a block diagram of a cluster of computer systems shown in FIG.1 in accordance with the preferred embodiments;

FIG. 3 is a flow diagram of a method in accordance with the preferredembodiments for powering up servers in a cluster;

FIG. 4 is a prior art method for a server to shut itself down based onthe loss of lock on a shared disk drive; and

FIG. 5 is a flow diagram of a method in accordance with the preferredembodiments for powering down unresponsive servers in a computer clusterbefore failing over the resources of the failed servers.

BEST MODE FOR CARRYING OUT THE INVENTION

According to preferred embodiments of the present invention, aquorum-based server power-down mechanism in a computer cluster assuresthat an unresponsive server in the cluster is powered-down before theresources are failed over to one or more other responsive servers. Thepower-down mechanism is quorum-based, meaning that only a cluster thatincludes a majority of the servers in the cluster may perform power-downoperations. By powering down failed servers, the preferred embodimentsassure that a failed system does not become responsive again.

The prior art provides a way for a server in a cluster to determine whenit has become unresponsive, and to know it needs to shut down. Method400 in FIG. 4 shows the steps in one known method in the art that uses ashared disk drive. When different computer systems in a cluster share adisk drive, there is typically a locking mechanism on the disk drive toassure only one server can access the disk drive at any given time. Aset of servers that are visible to each other using some membershipalgorithm will elect a leader and this leader will obtain the lock onthe disk drive. If the set of servers split into partitions because of acommunication fault, then the majority partition will obtain a lock onthe shared disk drive (step 410). A majority partition is determinedwith a voting system. This will cause the original leader to detect thatthe lock on the shared disk drive has been stolen (step 420=YES), andthe servers in the original partition will panic as a result (step 430).The panic may result in powering down the server or panicking theoperating system kernel.

The check for a majority partition is necessary because differentpartitions will realize that we have partitioned in an asynchronousmanner. If no partition had a majority then each partition will panicany servers with resources active. While method 400 in FIG. 4 issomewhat effective for servers that share a disk drive, the trend in theindustry is to get away from sharing resources between servers in acluster. In addition, some servers in a cluster may not need sharedstorage, making method 400 inapplicable to such servers. As a result, amethod is needed to know when a server fails, and to take appropriateaction to assure the server is dead when it is unresponsive.

Referring now to FIG. 1, a computer system 100 is one suitableimplementation of an computer system that may be a member of a clusterin accordance with the preferred embodiments of the invention. Computersystem 100 is an IBM eServer iSeries computer system. However, thoseskilled in the art will appreciate that the mechanisms and apparatus ofthe present invention apply equally to any computer system, regardlessof whether the computer system is a complicated multi-user computingapparatus, a single user workstation, or an embedded control system. Asshown in FIG. 1, computer system 100 comprises one or more processors110, a main memory 120, a mass storage interface 130, a displayinterface 140, a network interface 150, and a service processorinterface 180. These system components are interconnected through theuse of a system bus 160. Mass storage interface 130 is used to connectmass storage devices (such as a direct access storage device 155) tocomputer system 100. One specific type of direct access storage device155 is a readable and writable CD RW drive, which may store data to andread data from a CD RW 195.

Service processor interface 180 preferably connects the computer system100 to a separate service processor 182. Service processor 182preferably includes a server power-down mechanism 184 that allowsservers coupled to the service processor to be individuallypowered-down. Service processor 182 typically provides an interface thatallows a computer system (such as 100) to command the service processorto power down another computer system in the cluster. In addition,service processor 180 can terminate a single process on another machinewhen servers in the cluster are processes rather than physical boxes orlogical partitions.

Main memory 120 in accordance with the preferred embodiments containsdata 121, an operating system 122, and a cluster engine 123. Data 121represents any data that serves as input to or output from any programin computer system 100. Operating system 122 is a multitasking operatingsystem known in the industry as OS/400; however, those skilled in theart will appreciate that the spirit and scope of the present inventionis not limited to any one operating system. Cluster engine 123 providesfor communication between computer systems in a cluster. Cluster engine123 includes many features and mechanisms that are known in the art thatsupport cluster communications but are not shown in FIG. 1. Clusterengine 123 includes a heartbeat mechanism 124 possibly over multiplechannels, a membership change mechanism 125, and a quorum-based serverpower-down mechanism 126. The heartbeat mechanism 124 and membershipchange mechanism 125 are preferably known mechanisms in the art.Heartbeat mechanism 124 sends a periodic heartbeat message to otherservers in the cluster, and receives periodic heartbeat messages fromother servers in the cluster. These heart beats can be transmitted usinga variety of channels such as network, serial cables or shared diskbased heart beating. Membership change mechanism 125 monitors themembership in the cluster, and generates a membership change message toall servers in the cluster when one of the servers in the clusterbecomes unresponsive (i.e., stops sending heartbeat messages).Quorum-based server power-down mechanism 126 allows a manager server topower down unresponsive servers, thereby assuring that the unresponsiveservers do not become responsive in the future. The quorum-based serverpower-down mechanism 126 can only power down a server if the cluster hasquorum, as discussed in more detail below with reference to FIG. 5.

In computer system 100 of FIG. 1, the quorum-based server power-downmechanism 126 is shown to be part of the cluster engine 123. This,however, is shown only as one possible implementation within the scopeof the preferred embodiments. The quorum-based server power-downmechanism 126 could also be implemented separate from the cluster engine123. The preferred embodiments expressly extend to any suitable locationand implementation for the quorum-based server power-down mechanism 126.

Computer system 100 utilizes well known virtual addressing mechanismsthat allow the programs of computer system 100 to behave as if they onlyhave access to a large, single storage entity instead of access tomultiple, smaller storage entities such as main memory 120 and DASDdevice 155. Therefore, while data 121, operating system 122, and clusterengine 123 are shown to reside in main memory 120, those skilled in theart will recognize that these items are not necessarily all completelycontained in main memory 120 at the same time. It should also be notedthat the term “memory” is used herein to generically refer to the entirevirtual memory of computer system 100, and may include the virtualmemory of other computer systems coupled to computer system 100.

Processor 110 may be constructed from one or more microprocessors and/orintegrated circuits. Processor 110 executes program instructions storedin main memory 120. Main memory 120 stores programs and data thatprocessor 110 may access. When computer system 100 starts up, processor110 initially executes the program instructions that make up operatingsystem 122. Operating system 122 is a sophisticated program that managesthe resources of computer system 100. Some of these resources areprocessor 110, main memory 120, mass storage interface 130, displayinterface 140, network interface 150, system bus 160, and serviceprocessor interface 180.

Although computer system 100 is shown to contain only a single systembus, those skilled in the art will appreciate that the present inventionmay be practiced using a computer system that has multiple buses. Inaddition, the interfaces that are used in the preferred embodiment eachinclude separate, fully programmed microprocessors that are used tooff-load compute-intensive processing from processor 110. However, thoseskilled in the art will appreciate that the present invention appliesequally to computer systems that simply use I/O adapters to performsimilar functions.

Display interface 140 is used to directly connect one or more displays165 to computer system 100. These displays 165, which may benon-intelligent (i.e., dumb) terminals or fully programmableworkstations, are used to allow system administrators and users tocommunicate with computer system 100. Note, however, that while displayinterface 140 is provided to support communication with one or moredisplays 165, computer system 100 does not necessarily require a display165, because all needed interaction with users and other processes mayoccur via network interface 150.

Network interface 150 is used to connect other computer systems and/orworkstations (e.g., 175 in FIG. 1) to computer system 100 across anetwork 170. The present invention applies equally no matter howcomputer system 100 may be connected to other computer systems and/orworkstations, regardless of whether the network connection 170 is madeusing present-day analog and/or digital techniques or via somenetworking mechanism of the future. In addition, many different networkprotocols can be used to implement a network. These protocols arespecialized computer programs that allow computers to communicate acrossnetwork 170. TCP/IP (Transmission Control Protocol/Internet Protocol) isan example of a suitable network protocol.

At this point, it is important to note that while the present inventionhas been and will continue to be described in the context of a fullyfunctional computer system, those skilled in the art will appreciatethat the present invention is capable of being distributed as a programproduct in a variety of forms, and that the present invention appliesequally regardless of the particular type of signal bearing media usedto actually carry out the distribution. Examples of suitable signalbearing media include: recordable type media such as floppy disks and CDRW (e.g., 195 of FIG. 1), and transmission type media such as digitaland analog communications links.

Referring to FIG. 2, a simple cluster 200 of five computer systems (or“nodes”) is shown. Note that each node 100 in the cluster 200 ispreferably a computer system 100 as shown in FIG. 1. However, oneskilled in the art will recognize that different types of computersystems could be interconnected in a cluster. The connections betweennodes in FIG. 2 represent logical connections, and the physicalconnections can vary within the scope of the preferred embodiments aslong as the nodes in the cluster can logically communicate with eachother. Each node 100 is connected to a service processor 182. Theservice processor 182 preferably includes logic that allows forindividually powering down each server on each node. When a node incluster 200 becomes unresponsive, the quorum-based server power-downmechanism 126 in a manager server gives one or more commands to theservice processor 182 to power down one or more of the servers in thecluster 200. The service processor 182, in response to the command(s)from the manager server, powers down the one or more servers in thecluster. Note that the term “power down” and “powering down” denotesremoving power to the server, but can also denote simply putting theserver in a non-functional state using any suitable mechanism or means.For example, the service processor 182 could simply assert and hold ahard reset signal to a node that needs to be powered down. As long asthe reset signal is asserted, the node cannot power up. If a server islocated in a logical partition on an apparatus that includes otherservers in the cluster in one or more other logical partitions that arestill responsive, the apparatus cannot be physically powered downbecause this would reset the responsive servers as well. However, theservice processor can assert a signal or provide a command that causesthe server that needs to be powered off to instead shut down. Thus, theterm “power down” and “powering down” as used in this specification andclaims means any way, whether currently known or developed in thefuture, for putting a server in an unresponsive state until a supervisordetermines that the server may be powered back up. In addition, theseterms could also refer to simply restarting the server. A serviceprocessor may also be more fine grained, and if the members of thecluster were processes rather than physical boxes or logical partitions,then the powering down of the server may be the simple step ofguaranteeing the server process was terminated.

Referring to FIG. 3, a method 300 is a method in accordance with thepreferred embodiments for initially powering up servers in a cluster.The manager server is powered up first (step 310). This is done becausethe algorithms for powering down boxes when the manager server moves canreset boxes that are in the process of starting. This makes the initialbring up of the cluster much smoother. The rest of the servers in thecluster may then be powered up (step 320). For the sake of simplicity,in method 300 we assume there is a single manager server for a cluster.However, one skilled in the art will realize that multiple managerscould be defined for a cluster, with an arbitration scheme to determinewhich manager is responsible for performing management duties at anyparticular point in time. In the case of multiple manager servers, allmanager servers are started in step 310, followed by the servers thatare not managers in step 320.

FIG. 5 shows one specific method 500 that is preferably performed by thequorum-based server power-down mechanism 126 in FIG. 1 in accordancewith the preferred embodiments. Method 500 begins when one or moreservers in the cluster fail (step 510). If the cluster does not havequorum (step 520=NO), method 500 is done. The cluster has quorum if thecluster contains a majority of the servers in the cluster. Thus, acluster with seven servers that has three of the servers fail still hasquorum, but if four servers fail, the remaining cluster no longer hasquorum. If the number of possible servers is even then one server isgiven two votes and acts as a tiebreaker. One skilled in the art candetermine other techniques for creating tiebreakers. If the cluster hasquorum (step 520=YES), method 500 determines whether a manager serverfailed (step 530). Step 530 does not simply test to see if a manager hasever failed, but more specifically tests to see if a manager server isone of the servers that failed to start method 500 in step 510. If themanager server failed in step 510 (step 530=YES), all non-visibleservers in the cluster that have a critical resource are powered down(step 540). A server is non-visible in the cluster (i.e., unresponsive)if it has stopped sending heartbeat messages, or if it has beenpartitioned from the cluster. If no manager server failed (step 530=NO),method 500 powers down the servers that failed in step 510 thatcurrently are potential owners of any quorum protected resource. Thischeck is critical as it allows a server process to be shutdown cleanlyand it won't be powered down as a result. The difference between steps540 and 550 is simply this: if a manager fails, we don't necessarilyknow which failed node used to be the manager, so we must power down allunresponsive servers in the cluster (step 540) to avoid the managercoming back alive in the future. If the manager does not fail, only thefailed servers that can potentially own a quorum protected resource needto be powered down (step 550).

If the power-down operation succeeded (step 560=YES)), the resources onthe failed server(s) may be failed over to servers in the cluster thatare still responsive (step 570). The concept of failing over resourcesfrom a dead server to a live server in the cluster is well-known in theart, and therefore need not be discussed in further detail here. Thefailing over of resources is the process of making these same resourcesavailable on a different server in the cluster. This is the very natureof one specific way to provide highly-reliable services, using multipleservers that can take over for each other when one of the servers fails.If the power-down operation did not succeed (step 560=NO), the clusteris disabled (step 580). The preferred embodiments depend on the serviceprocessor doing its job of powering down a selected server when thequorum-based server power-down mechanism sends the command to power downthe selected server. If the service processor is unable to perform itspower-down function, this means there is a problem with the serviceprocessor itself or something else that requires intervention by asystem administrator. Thus, once a cluster is disabled in step 580, asystem administrator is preferably notified of the problem so the systemadministrator can take appropriate action to correct the problem.

With an understanding of method 500 in FIG. 5, we now understand why itis necessary to power up the manager server first in method 300 of FIG.3 before powering up the other servers. Let's assume a manager server Bwas powered up after another server A is powered up. In this scenario,when A powers up, it will assume it is the manager. When server C powersup, it will detect a change in manager server, which it will interpretas a failure of the previous manager, and will power down allnon-visible servers. These non-visible servers may be in the process ofpowering up, and each time the manager changes, they are effectivelykilled off before they can complete the power-up sequence. By requiringthe manager server to be powered up first (step 310), followed by theother servers (step 320), this type of undesirable behavior is avoided.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. Thus, while the invention hasbeen particularly shown and described with reference to preferredembodiments thereof, it will be understood by those skilled in the artthat these and other changes in form and details may be made thereinwithout departing from the spirit and scope of the invention. Forexample, while a known service processor is shown as one possiblemechanism for powering down servers, other mechanisms could also be usedwithin the scope of the preferred embodiments. For example, addressablepower strips could be used that are capable of receiving commands, andshutting off power to a particular plug in the power strip or to theentire power strip. Any mechanism for putting a server in anunresponsive state until some step of intervention is taken falls withinthe scope of the term “service processor” as used herein. In addition,the servers recited herein may reside within logical partitions, whichmeans that the power down of a server in a logical partition impliessimply shutting down the logical partition.

1. An apparatus comprising: at least one processor; a memory coupled tothe at least one processor; a server process residing in the memory andexecuted by the at least one processor; a cluster engine residing in thememory and executed by the at least one processor, the cluster enginehandling communications between the server process and other servers ina cluster; and a quorum-based server power-down mechanism residing inthe memory and executed by the at least one processor, the quorum-basedserver power-down mechanism determining whether the server process ispart of a group of servers that include a majority of servers in thecluster, and if so, the quorum-based server power-down mechanismdetermines whether a manager of the cluster failed when an indication ofa server failure is received, and if so, the quorum-based serverpower-down mechanism issues at least one command to power down allunresponsive servers in the cluster, wherein an unresponsive server is aserver that fails to send a periodic message that indicates the serveris functioning properly, and if the manager of the cluster did not fail,the quorum-based server power-down mechanism issues at least one commandto power down a server corresponding to the received indication ofserver failure.
 2. The apparatus of claim 1 further comprising a serviceprocessor that receives the command, and in response, powers down the atleast one of the other servers.
 3. The apparatus of claim 1 wherein thecluster engine comprises a heartbeat mechanism that sends a periodicmessage to the other servers in the cluster to indicate the serverprocess is functioning properly.
 4. The apparatus of claim 3 wherein theheartbeat mechanism receives periodic messages from the other servers inthe cluster that indicate the other servers in the cluster arefunctioning properly.
 5. The apparatus of claim 4 wherein the clusterengine comprises a membership change mechanism that generates amembership change message to all servers in the cluster when a selectedserver in the cluster becomes unresponsive by failing to send a periodicmessage that indicates the selected server is functioning properly. 6.The apparatus of claim 1 wherein the cluster engine comprises amembership change mechanism that generates a membership change messageto all servers in the cluster when any of the servers in the clusterbecome unresponsive.
 7. The apparatus of claim 1 wherein the serverprocess resides in a logical partition defined on the apparatus.
 8. Theapparatus of claim 1 wherein the quorum-based server power-downmechanism determines whether the power down of the at least one of theother servers was successful, and if the power down of the at least oneof the other servers was successful, the quorum-based server power-downmechanism enables failing over any resources on the at least one of theother servers that was powered down to at least one server that isresponsive, and if the power down of the at least one of the otherservers was not successful, the quorum-based server power-down mechanismdisables the cluster.
 9. A networked computer system comprising: aplurality of servers coupled together via a network into a cluster, eachserver comprising: a cluster engine that handles communications betweenservers in the cluster; and a quorum-based server power-down mechanismthat determines whether a server is part of a group of servers thatincludes a majority of servers in the cluster, and if so, thequorum-based server power-down mechanism determines whether a manager ofthe cluster failed when an indication of a server failure is received,and if so, the quorum-based server power-down mechanism issues at leastone command to power down all unresponsive servers in the cluster,wherein an unresponsive server is a server that fails to send a periodicmessage that indicates the server is functioning properly, and if themanager of the cluster did not fail, the quorum-based server power-downmechanism issues at least one command to power down a servercorresponding to the received indication of server failure.
 10. Thenetworked computer system of claim 9 further comprising a serviceprocessor coupled to each of the plurality of servers in the cluster,the service processor receiving the command, and in response, poweringdown the at least one server in the cluster.
 11. The networked computersystem of claim 9 wherein the cluster engine comprises a heartbeatmechanism that sends a periodic message to other servers in the clusterto indicate the server is functioning properly.
 12. The networkedcomputer system of claim 11 wherein the heartbeat mechanism receivesperiodic messages from the other servers in the cluster that indicatethe other servers in the cluster are functioning properly.
 13. Thenetworked computer system of claim 12 wherein the cluster enginecomprises a membership change mechanism that generates a membershipchange message to all servers in the cluster when a selected server inthe cluster becomes unresponsive by failing to send a periodic messagethat indicates the selected server is functioning properly.
 14. Thenetworked computer system of claim 9 wherein the cluster enginecomprises a membership change mechanism that generates a membershipchange message to all servers in the cluster when any of the servers inthe cluster become unresponsive.
 15. The networked computer system ofclaim 9 wherein at least one server in the cluster resides in a logicalpartition.
 16. The networked computer system of claim 9 wherein thequorum-based server power-down mechanism determines whether the powerdown of the at least one of the other servers was successful, and if thepower down of the at least one of the other servers was successful, thequorum-based server power-down mechanism enables failing over anyresources on the at least one of the other servers that was powered downto at least one server that is responsive, and if the power down of theat least one of the other servers was not successful, the quorum-basedserver power-down mechanism disables the cluster.
 17. A computerreadable recordable medium bearing a computer program, the computerprogram comprising: a quorum-based server power-down mechanism thatdetermines whether a server is part of a group of servers that include amajority of servers in a cluster, and if so, the quorum-based serverpower-down mechanism determines whether a manager of the cluster failedwhen an indication of a server failure is received, and if so, thequorum-based server power-down mechanism issues at least one command topower down all unresponsive servers in the cluster, wherein anunresponsive server is a server that fails to send a periodic messagethat indicates the server is functioning properly, and if the manager ofthe cluster did not fail, the quorum-based server power-down mechanismissues at least one command to power down a server corresponding to thereceived indication of server failure.
 18. The computer readablerecordable medium of claim 17 wherein the computer program furthercomprises a heartbeat mechanism that sends a periodic message to theother servers in the cluster to indicate the server process isfunctioning properly.
 19. The computer readable recordable medium ofclaim 18 wherein the heartbeat mechanism receives periodic messages fromthe other servers in the cluster that indicate the other servers in thecluster are functioning properly.
 20. The computer readable recordablemedium of claim 19 wherein the computer program further comprises amembership change mechanism that generates a membership change messageto all servers in the cluster when a selected server in the clusterbecomes unresponsive by failing to send a periodic message thatindicates the selected server is functioning properly.
 21. The computerreadable recordable medium of claim 17 wherein the computer programfurther comprises a membership change mechanism that generates amembership change message to all servers in the cluster when any of theservers in the cluster become unresponsive.
 22. The computer readablerecordable medium of claim 17 wherein at least one server in the clusterresides in a logical partition.
 23. The computer readable recordablemedium of claim 17 wherein the quorum-based server power-down mechanismdetermines whether the power down of the at least one of the otherservers was successful, and if the power down of the at least one of theother servers was successful, the quorum-based server power-downmechanism enables failing over any resources on the at least one of theother servers that was powered down to at least one server that isresponsive, and if the power down of the at least one of the otherservers was not successful, the quorum-based server power-down mechanismdisables the cluster.